In accordance with the recent miniaturization of electronics devices, it is required to increase electric capacitance of an aluminum foil for electrolytic capacitor electrodes to be built in electronics devices.
In general, an aluminum material for electrolytic capacitor electrodes is subjected to etching treatment in order to increase the surface expansion ratio of the aluminum material to thereby increase the electric capacitance. The deeper the depth of etching pit formed by etching treatment is, the higher the surface expansion ratio becomes. Therefore, various proposals for improving etching characteristics have been made. For example, regarding manufacturing steps, various treatments, such as degreasing washing treatment before final annealing, hydration treatment before final annealing and crystalline oxide film forming treatment at the final annealing or oxidation treatment before final annealing, have been proposed (see Japanese Examined Laid-open Patent Publication No. S58-34925, Japanese Unexamined Laid-open Patent Publication No. 3-122260, etc.). Furthermore, regarding compositions, there have been proposed to slightly add elements such as Pb or Bi or to add elements such as Ti, Zr and/or V to thereby make these elements condense in the surface layer of the aluminum material (Japanese Unexamined Laid-open Patent Publication No. 4-62822). Furthermore, regarding a crystal structure, there has been proposed to adjust the (100) crystal orientation.
However, merely increasing the depth of each etching pit was not enough to improve the surface expansion ratio of the aluminum foil. In order to further increase the surface expansion ratio, it is necessary to generate etching pits uniformly on the etching surface with high density by decreasing local etching, un-etching and surface dissolution. According to the aforementioned various methods and/or compositions, it was not enough to generate etching pits uniformly with high density. Therefore, the aforementioned methods and/or compositions do not meet requests of increasing electric capacitance.